FIX: Better infinite scrolling on categories page (#24831)

This commit refactor CategoryList to remove usage of EmberObject,
hopefully make the code more readable and fixes various edge cases with
lazy loaded categories (third level subcategories not being visible,
subcategories not being visible on category page, requesting for more
pages even if the last one did not return any results, etc).

The problems have always been here, but were not visible because a lot
of the processing was handled by the server and then the result was
serialized. With more of these being moved to the client side for the
lazy category loading, the problems became more obvious.

spec/models/category_list_spec.rb

@@ -374,4 +374,18 @@ RSpec.describe CategoryList do
       expect(category_list.categories[-1].custom_field_preloaded?("bob")).to be_falsey
     end
   end
+
+  describe "lazy_load_categories" do
+    fab!(:category) { Fabricate(:category, user: admin) }
+    fab!(:subcategory) { Fabricate(:category, user: admin, parent_category: category) }
+
+    before { SiteSetting.lazy_load_categories = true }
+
+    it "returns categories with subcategory_ids" do
+      expect(category_list.categories.size).to eq(3)
+      expect(
+        category_list.categories.find { |c| c.id == category.id }.subcategory_ids,
+      ).to contain_exactly(subcategory.id)
+    end
+  end
 end